For example, in a printer, a paper feed cassette for storing papers is detachably mounted to a printer main body. The paper feed cassette includes a driving gear, which is coupled with an output gear of a device main body at the time of being mounted to the printer main body so as to push up a stacking plate on which sheets are stacked, and a pushing up member that lifts the stacking plate from below by the rotation of the driving gear fixed to an end of a rotating shaft of the driving gear. In this case, a driving force of the output gear is transmitted to the driving gear when starting the image forming operation of the printer, and thus the stacking plate is lifted so that the uppermost paper is lifted to a position at which a paper can be fed by a paper feeder of the printer main body.
Here, since the paper feed cassette is attached and detached, the driving gear is coupled with and is released from the output gear of the main body in each case. For this reason, when the paper feed cassette is mounted to the printer main body, the tooth tips of the output gear and the driving gear interfere with each other. Accordingly, the teeth of both gears do not smoothly engage with each other. A technology for smoothly engaging the gears with each other is disclosed in JP-UM-A-5-10854. In the technology, since each of the tooth tips of a gear mounted to a cover is made to be pointed at the end, the cover is smoothly opened and closed.